The present invention relates to integrated field-emission elements operable at a low voltage and to methods of forming such elements.
The fabrication of miniaturized field-emission elements became possible by the advancements of semiconductor fabrication technologies. In particular, Spindt et al. disclosed the fabrication of a corn-shaped (vertical) field-emission cathode. (C. A. Spindt, J. Appl. Phys, Vol. 47, p. 5248 (1976).
Turning now to the drawings, FIG. 12(a)-FIG. 12(d) depict the conventional fabrication method of a field-emission cathode disclosed by Spindt et al. The Spindt et al. process is explained below.
As depicted in FIG. 12(a), the fabrication process is begun with depositions of an insulation layer 101 and a metal layer 102 utilized as a gate electrode on a semiconductor (silicon) substrate 100. A round small hole 103 is then formed in said metal layer 102 and insulation layer 101 by using a conventional photolithographic process.
As depicted in FIG. 12(b), a sacrificing layer 104, made of a material such as alumina, is vacuum deposited on the semiconductor substrate 100 at a shallow angle thereto and the gate electrode. As a result, the diameter of gate hole 103 is substantially reduced. Then, as shown in FIG. 12(c), metal layer 105, made of a material such as molybdenum, is vertically deposited on semiconductor substrate 100. The gate-hole diameter is gradually reduced as the metal layer 105 is vacuum deposited, and a cone-shaped emitter (cathode) 106 is formed within gate hole 13.
The fabrication process is completed by removing the sacrificing layer 104 and the unnecessary metal layer 105. The field-emission cathode, thus obtained, is operable by applying a high-voltage on gate electrode 102. This causes electrons to be drawn into a vacuum from emitter 106. The electrons are collected by an anode (not shown) disposed at a position opposing emitter 106.
Following the fabrication process disclosed by Spindt et al., a vertical type field-emission cathode of similar construction, having a sharper emitter formed by applying either an anisotropic etching or a thermal oxidation process on a silicon crystal surface was disclosed. (H. F. Gray et al., IEDM Tech. Dig. P. 776 (1986), and Betsui, Trans. 1990 Fall. Conv. of Elect. Inform. Comm. Engineer. Japan, No. 5, SC-8-2 (1990)).
In contrast to the fabrication of vertical structure cathodes described above, Itoh et al. disclose a field-emission cathode of planar construction (Itoh et al., Vacuum. Vol. 34, P. 867 (1991)). As depicted in FIG. 13(a), the planar field-emission cathode is shown as a comb-shaped emitter 108 made of an etched-off metal layer disposed on quartz substrate 107, gate 109, and anode (not shown) deposited on the same substrate. The planar cathode disclosed by Itoh et al. has small capacitances, and is highly advantageous for use in various ultra high-speed electron devices. This is particularly true for the devices employing a silicon substrate instead of the quartz substrate, since it can be integrated with Large Scale Integration (LSI) devices. The method of fabricating planar field-emission cathodes is explained below in connection with FIG. 13(b)FIG.13(h).
Turning now to FIG. 13(b), a cathode metal layer 108 made of tungsten (W) is deposited first on quartz substrate 107. Then, as shown in FIG. 13(c), the outline of emitter layer 108 is drawn by RIE (reactive ion etching) using a photoresist layer 109 as a mask. Then, the quarter substrate 107 is etched off into a form shown in FIG. 13(d) by using fluoric acid. After vacuum depositing a gate metal 110 thereon, as shown in FIG. 13(e), a wet-etching is applied thereon to form gate electrode 110, and the photoresist layer 109, deposited on emitter 108, is removed.
Then, as shown in FIG. 13(f), gate electrode 110 is formed by employing serial photolithographic and wet-etching processes in which a photoresist layer 111 is deposited thereon and is used as a mask. As shown in FIG. 13(g) and FIG. 13(h), a comb-shaped emitter 112 is formed by applying serial photolithographic and wet-etching processes utilizing photoresist layer 113 as a mask.
However, there are problems associated with the conventional cone-shaped field-emission cathodes, and the process used to fabricate such cone-shaped cathodes. In particular, the minimum radius of curvature of the emitter and the emitter-to-gate distance are about 20 nm and 0.5 .mu.m, respectively. Also, an electron-beam exposure method has to be employed to ensure the uniformity of the curvatures of the emitter.
As for the comb-shaped (emitter) cathodes described above, such cathodes can be fabricated using a conventional photolithographic process. Also, the emitter-to-gate electrode distance is easily controllable to a submicron order, and the reproducibility and device uniformity are advantageously high. However, there are problems associated with the comb-shaped field-emission cathodes also. In particular, the minimum radius of curvature of the emitter available by the process disclosed by Itoh et al. is as large as 40 nm, and the operating field requires a relatively high voltage or electric field of 150V.
None of the prior art has the advantages of providing a field emission element with a minimum radius of curvature and minimum emitter-to-gate distances, operable at less than 150 volts, and fabricated by a process that results in high reproducibility, device uniformity and excellent characteristics.